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Message-ID: <17537d7f-8734-2186-b27c-f39f3110ffe5@redhat.com>
Date: Mon, 30 Jan 2023 14:57:55 -0500
From: Waiman Long <longman@...hat.com>
To: Qais Yousef <qyousef@...alina.io>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
Juri Lelli <juri.lelli@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>, tj@...nel.org,
linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
claudio@...dence.eu.com, tommaso.cucinotta@...tannapisa.it,
bristot@...hat.com, mathieu.poirier@...aro.org,
Dietmar Eggemann <dietmar.eggemann@....com>,
cgroups@...r.kernel.org,
Vincent Guittot <vincent.guittot@...aro.org>,
Wei Wang <wvw@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
Quentin Perret <qperret@...gle.com>
Subject: Re: [PATCH v2] sched: cpuset: Don't rebuild sched domains on
suspend-resume
On 1/30/23 14:48, Qais Yousef wrote:
> On 01/30/23 11:29, Waiman Long wrote:
>> On 1/30/23 08:00, Qais Yousef wrote:
>>
>> just skip the call here if the condition is right? Like
>>
>> /* rebuild sched domains if cpus_allowed has changed */
>> if (cpus_updated || (force_rebuild && !cpuhp_tasks_frozen)) {
>> force_rebuild = false;
>> rebuild_sched_domains();
>> }
>>
>> Still, we will need to confirm that cpuhp_tasks_frozen will be cleared
>> outside of the suspend/resume cycle.
>>
>> I think it's fine to use this variable from the cpuhp callback context only.
>> Which I think this cpuset workfn is considered an extension of.
>>
>> But you're right, I can't use cpuhp_tasks_frozen directly in
>> rebuild_root_domains() as I did in v1 because it doesn't get cleared after
>> calling the last _cpu_up().
>>
>> That is what I suspect. So we can't use that cpuhp_tasks_frozen variable here
>> in cpuset.
>>
>> force_rebuild will only be set after the last cpu
>> is brought online though - so this should happen once at the end.
>>
>> Perhaps you can add another tracking variable for detecting if suspend/resume
>> is in progress.
> I think cpuhp_tasks_frozen is meant for that. All users who cared so far
> belonged to the cpuhp callback. I think reading it from cpuset_hotplug_workfn()
> is fine too as this function will only run as a consequence of the cpuhp
> callback AFAICS. cpuset_cpu_active() takes care of not forcing a rebuild of
> sched_domains until the last cpu becomes active - so the part of it being done
> once at the end at resume is handled too.
Well we will have to add code to clear cpuhp_tasks_frozen at the end of
resume then. We don't want to affect other callers unless we are sure
that it won't affect them.
>
> It's just rebuild_sched_domains() will always assume it needs to clear and
> rebuild deadline accounting - which is not true for suspend/resume case. But
> now looking at other users of rebuild_sched_domains(), others might be getting
> the hit too. For example rebuild_sched_domains_locked() is called on
> update_relax_domain_level() which AFAIU should not impact dl accounting.
>
> FWIW, I did capture a worst case scenario of 21ms because of
> rebuild_root_domains().
>
> /me thinks rebuild_root_domains() is a misleading name too as it just fixes
> dl accounting but not rebuild the rd itself.
>
> What makes sense to me now is to pass whether dl accounting requires updating
> to rebuild_sched_domains() as an arg so that the caller can decide whether the
> reason can affect dl accounting.
>
> Or maybe pull rebuild_root_domains() out of the chain and let the caller call
> it directly. And probably rename it to update_do_rd_accounting() or something.
>
> I'll continue to dig more..
Looking forward to see that.
Cheers,
Longman
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